fix: upgrade openapi-generator-cli version to 6.6.0 for elixir

[tobbbles]

Fixes #282.

Related Issue or Design Document

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [x] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added the necessary documentation within the code base (if appropriate).

Further comments

There are still some annoying bugs and inconsistencies in the Elixir implementation of openapi-generator. My fingers are crossed for version 7.0.0 and the following PR https://github.com/OpenAPITools/openapi-generator/pull/15746, which will introduce some breaking changes to any users currently using the Elixir clients, but brings massive improvements and overhauls.